Field
This disclosure relates generally to semiconductor devices, and more specifically, to protecting copper wire bonds in the semiconductor device.
Related Art
As the price of gold has seen a sharp increase in value over the past few years, the cost of producing semiconductors with gold wire bonds has increased proportionally. Semiconductor designers are searching for lower cost materials in order to control costs while maintaining or improving reliability of the semiconductor devices.
Copper is becoming a more popular choice as an interconnection material in semiconductor packaging because the cost of copper is a fraction of the cost of gold. Copper also offers superior electrical and thermal conductivity, develops less intermetallic growth, has greater reliability of the bond at elevated temperatures, and has higher mechanical strength and stability.
Copper wire bonding has not been a popular choice because it is more stressful to a typical aluminum bond pad than gold wire bonding, and the risk of physical damage to aluminum bond pad is greater. This is due to greater hardness of copper versus gold and the need to use more severe wire bonding parameters (e.g. higher force, higher power, higher temperature) for copper wire bonding due to the greater hardness of copper and the slower growth of intermetallic layers versus gold-aluminum intermetallic layers. From the 90 nanometer integrated circuit technology node onward, the back end of line (BEOL) layer stack has been comprised of low dielectric constant (low K) dielectric and copper layers, with a tantalum nitride or tantalum barrier layer between the dielectric and the copper. The bond pad damage risk with copper wire bonding has become more severe with each new IC technology node incorporating increasingly lower dielectric constant interlayer dielectrics (ILD), and copper interconnect. The progressively lower strength of these low K dielectrics and the relatively low adhesion strength between materials comprising the ILD-copper stack are the causes of the increased risk of damage during wire bonding.
The risk of pad damage is also a function of bond pad design features. Increasing the aluminum pad thickness is known to reduce the risk of pad damage, but this can also increase the lateral displacement of aluminum (“aluminum push-out”) during the bonding process, risking damage to the passivation layer, or even shorting between very fine pitch bond pads.